The present invention concerns a device for detecting movements of a surface of an object, particularly for a computer pointing tool, including a light source, guide means for directing light coming from said source obliquely over a field of said surface, an optoelectronic sensor provided with an array of photo-sensors, an optical system for forming an image of said field on the optoelectronic sensor.
U.S. Pat. No. 6,124,587 discloses a pointing device with a ball wherein the movements of the surface of the ball by rotation of the latter are detected by optical means, the surface being provided with a pattern formed of regular or irregular spots. A small portion of the surface of the ball, forming a field defined by a fixed window in which one wishes to detect the movements of the ball surface, is illuminated obliquely by means of two light emitting diodes (LED) mounted in a support on either side of a lens. The light reflected by the field thereby illuminated is reflected by a mirror associated with the lens towards an optoelectronic sensor including an array of photo-sensors. The signals originating from the sensor are processed by electronic circuits mounted on a printed circuit board, to provide data relating to the movement of the field along two components in its plane.
The construction illustrated in particular by FIGS. 7 to 12 of the aforementioned Patent, includes an optical housing in two parts used as a mounting piece for the LED's, the mirror, the lens and the optoelectronic sensor. The mirror is formed by a reflective surface on a plane face of the lens. The housing is secured by being snap fitted, on the one hand, onto the ball guide housing and on the other hand, onto the printed circuit board. The sensor and the LED's include metal connecting lugs to connect them electrically to the printed circuits of the board.
In an arrangement of this type, two light sources have to be used, at a distance from each other, in order to obtain sufficiently uniform illumination of the observed field, so that the detection of its movements can occur in proper conditions. The light originating from the LED's is led to this field by respective channels arranged in the optical housing. Consequently, the housing is relatively complicated and several groups of connecting lugs are necessary for connecting the LED's and the printed circuit board.
Another drawback of this construction lies in the cost of the mirror, the role of which is to obtain a bend path for the light from the observed field to the sensor, to prevent the latter from receiving stray light originating from elsewhere.
Other drawbacks are linked to the fact that the optical housing, assuring precise mutual positioning of all the components which it carries, constitutes a complicated part requiring an expensive mould for mass production. If one of the components, or its position, has to be modified, a new mould has to be made, involving significant costs. Moreover, this construction requires a step of making numerous electric connections to the printed circuit board after the optical housing has been mounted on the board, which complicates manufacturing.
U.S. Pat. No. 5,686,720 discloses a construction, which avoids certain of the aforementioned drawbacks. It is a so-called navigation sensor, incorporated in a hand-held scanner for measuring the movements of a scanner over a document. In the example illustrated by FIG. 5 of said patent, a small field of the document is illuminated by a light source through a prism placed just above it. The prism deviates the light beam so that the light reaches the paper at a small angle, typically less than 16°. The light diffused by the paper passes back through the prism and is collimated towards an array of photo-sensors by means of a convex surface of the prism, forming a lens. This enables the light source and the optoelectronic sensor with photo-sensors to be mounted directly on the printed circuit board. However, this arrangement does not allow the sensor to be efficiently protected against the stray light originating from the prism region.